Motion Graphs 3

Motion Graphs - Part III

Objectives

• Be able to sketch or plot speed-time graphs for objects moving at constant speed and constant acceleration.

• Be able to interpret a speed-time graph in order to describe the motion of the object shown.

• Be able to calculate acceleration and distance travelled from a speed/time graph.

Task 1 - Recap

The motion graphs we have looked at so far have all be distance-time graphs. Your teacher will give you a copy of the graph and questions below. Stick the graph into your exercise book and answer the questions.

(a) Describe the motion in words.

(b) Calculate the speed in the first 4 seconds.

(d) Calculate the average speed.

Task 2

We now need to look at a second type of motion graph - a speed-time graph.

Look at this presentation to see how speed-time graphs are plotted and how we can interpret the information on the graph.

Task 3

You will need to practise drawing distance time graphs. Read the information below and plot a speed-time graph of the data given. Your teacher will give you a copy of the information to stick in your exercise book.

A cyclist sets off and accelerates from rest to a speed of 12m/s, taking 6 seconds to reach this speed. He maintains this speed for 30 seconds. The cyclist then accelerates again, taking 8 seconds to reach a speed of 16m/s. He maintains this speed for a further 20seconds, then decelerates to a stop, taking 10 seconds to come to rest.

Teacher note: here is a completed version of the graph in excel format.

Task 4

We need to learn two rules, which will allow us to find out the distance travelled by an object and the rate of acceleration for objects which are not travelling at a constant speed.

The gradient of a speed-time graph is the acceleration.

and

The area under a speed-time graph is the distance travelled.

Learn both of these - they are not given!

Now, use the rules you have learned to answer these questions. Copy each question into your book and show full working for your calculations.

1) What is the speed of the cyclist after:

a) 2 seconds? b) 10 seconds? c) 40 seconds? d) 70 seconds?

2) What is the acceleration of the cyclist between 2 and 4 seconds?

3) What is the acceleration of the cyclist between 20 and 30 seconds?

4) What is the acceleration of the cyclist between 40 and 44 seconds?

5) What is the acceleration of the cyclist between 64 and 70 seconds?

6) What is the distance travelled by the cyclist in the first 6 seconds?

7) What is the total distance travelled by the cyclist?

Homework

Complete this worksheet.